1. The Field of the Invention
The present invention relates to devices for creating and maintaining vacuums. In particular, the present invention relates to a device for maintaining a vacuum in a thermally insulating jacket and to a process for manufacturing such a device.
2. The Background Art
Vacuum jackets are commonly employed for the storage and transportation of temperature-sensitive materials, such as cryogenic fluids (e.g., liquid nitrogen), and the creation and maintenance of controlled-temperature devices (e.g., refrigerators). Often, such vacuum jackets contain additional insulating materials such as glass wool, colloidal silica, perlite, or organic polymers in the form of foams (e.g., open, cell-rigid polyurethanes) or multilayers (i.e., alternating sheets of organic polymers, such as polyolefins, preferably cross-linked polyolefins, and aluminized plastics).
Unfortunately, the vacuum present initially in a vacuum jacket deteriorates as a consequence of various phenomena. For example, deterioration of the vacuum can result from the ingress of gases such as CO, CO.sub.2, O.sub.2, H.sub.2, H.sub.2 O, or N.sub.2 into the interior vacuum space of the jacket, usually by absorption at the welding joints of the walls of the jacket. In addition, organic vapors can be released from the above-described insulating materials, or from the materials forming the walls of the jacket. Also, in cases where the walls of the vacuum jacket are made of plastic, permeation through the jacket walls of gases, in particular, atmospheric gases, often occurs.
The deterioration of the jacket's interior vacuum can be alleviated at least partially by placing into the interior of the vacuum jacket materials capable of abstracting one or more of the above-listed gases. These materials are described generally as physical or chemical sorbers. Physical sorbers act according to the principle of physical adsorption in which gases adhere to the surface of the sorbing material as a result of weakly attractive forces between the material surface and the gas molecule. Materials that function as physical sorbers include, among others, zeolites, molecular sieves, and activated carbon. However, the sorption of gases by physical sorbers is generally reversible if the adsorbing material undergoes an increase in temperature. Thus, physical sorbers are not particularly well suited for vacuum jackets used to provide thermal insulation as such jackets are subject to changes in temperature during their use. For example, dewars used for the containment of liquid gases, such as liquid nitrogen, must be capable of withstanding changes in temperature between room temperature, typically the temperature of the empty dewar, and about -196.degree. C., the temperature of liquid nitrogen.
More preferable sorbing materials for use in vacuum jackets are chemical sorbers, materials that sorb and fix gases irreversibly by chemical reaction. Example of such materials include drying agents that are effective to abstract water vapor (H.sub.2 O), and getter materials that can isolate gases such as carbon monoxide (CO), carbon dioxide (CO.sub.2), molecular oxygen (O.sub.2), molecular hydrogen (H.sub.2), H.sub.2 O, or molecular nitrogen (N.sub.2), in addition to water vapor.
The use of chemical sorbers is described, for example, in PCT Application Serial No. WO 93/25843, incorporated herein by reference. This publication describes the combination of a drying agent, selected from barium oxide, strontium oxide, phosphorus oxide and their mixtures, and a getter material comprising an alloy of barium and lithium, and, in particular, the barium-aluminum alloy having the stoichiometric formula BaLi.sub.4. Although BaLi.sub.4 has the capacity to sorb each of the gases indicated above, the great affinity of BaLi.sub.4 for water vapor weakens its ability to abstract other gases. Hence, at least one of the above-described drying compounds is added to remove water vapor from the mixture of gases coming into contact with the getter material to preserve the sorption capacity of the getter material for other gases. Optionally, an oxide of a noble metal, preferably palladium oxide (PdO), can be included with the getter material and drying agent to convert traces of H.sub.2 into H.sub.2 O which is then fixed by the drying compound.
Also mentioned in the above-identified PCT application, in quite general terms, is the possibility that the drying agent and getter material might be placed inside a container and separated by a porous septum to form a device that can be used to maintain a vacuum. However, this disclosure does not account for the practical problems of preparing and handling such a device. In particular, at room temperature the drying agent and getter materials described in the above-identified PCT application are highly reactive towards the ambient atmosphere and must be isolated until their use. Also, these materials must be activated by heating to temperatures as high as 150.degree. C. while the jacket is maintained at a pressure of less than 5 Pascals (Pa). Such activation conditions can induce the release of gases into the vacuum space from jacket walls made of plastic.
U.S. Pat. No. 5,191,980 describes a mechanical device for protecting the sorbing materials from the environment until their use. The device holds a getter material hermetically sealed by a film of thermo-retractable material, such as a vinyl chloride resin. The getter material is exposed to the inner atmosphere of the jacket by heating to cause retraction and rupture of the thermo-retractable film. Again, however, the thermal treatment necessary to retract the thin film may not be compatible with the materials that comprise the jacket. Therefore, it would be advantageous to provide a device for maintaining a vacuum in a thermally insulating jacket that does not require heat activation. It would also be advantageous to provide a device that can be exposed to the atmosphere for a reasonable period of time and retain its effectiveness.